1. Field of the Invention
The invention relates to a liquid fuel direct oxidation fuel cell, especially, direct methanol fuel cell (DMFC) and in particular to a fuel supply control method for a fuel cell system, controlling fuel supply rate to fuel unit to maintain the fuel cell operating at acceptable lowest fuel concentration according to control a system temperature of the fuel cell system.
2. Description of the Related Art
As fuel is consumed in a liquid fuel direct oxidation fuel cell, not only volume but concentration of the fuel is decreased, with optimum, operation requiring control the system operate at lowest acceptable fuel concentration and appropriate temperature.
Conventional fuel supply methods for fuel cells, such as those disclosed in U.S. Pat. Nos. 6,589,671B1, 6,488,837, US2002/076589A1, and WO01/35478, supply fuel or water according to fuel concentration measured from a fuel cell. Another fuel supply method for fuel cells, disclosed by Ballard Power in U.S. Pat. No. 6,698,278B2, the fuel concentration was calculated using fuel input and output stream temperature and electrical current measured from a fuel cell. For these control method, accurate fuel concentration measurement or estimation is the basic requirement for controlling the system to operate at suitable fuel concentration. The fuel supply amount is calculated from the concentration discrepancy.
FIG. 1 is a block diagram of a conventional fuel cell 10 comprising a fuel supply control system 11, a fuel cell module 111, a pump 112, and a fuel unit 118 storing fuel with concentration maintained within a certain range. Pump 112 delivers the fuel to fuel cell module 111 for reaction with an anode.
Fuel supply control system 11 controls fuel concentration in fuel unit 118, and comprises a concentration meter 117, a controller 115, and a fuel supply device 12. Concentration meter 117 detects fuel concentration in fuel unit 118. Controller 115 calculates the difference between the measured concentration and a standard concentration, and drives fuel supply device 12 to deliver high concentration fuel to fuel unit 118 according to the calculated fuel amount from concentration difference, such that fuel concentration in fuel unit 118 is maintained at a standard.
Fuel supply device 12 comprises a fuel tank 113 storing high concentration fuel and a fuel deliver device 114 delivering fuel from fuel tank 113 to fuel unit 118. In other designs, a water tank or a fuel tank storing low concentration fuel may be required if fuel concentration in fuel tank 113 is too high. However, in general, recycled water from a cathode unit of the fuel cell system is sufficient for the same function.
Devices for detection of the fuel concentration suffer: difficulty in achieving accuracy and high cost. For example, apparatus disclosed in U.S. Pat. No. 6,589,679 utilizes another fuel cell to measure the fuel concentration in a main fuel cell, whereby various factors, such as temperature change, impurities in fuel, and age of devices, may affect measurement accuracy. That disclosed by Ballard Power provides another solution in the field, although complex functions must be generated based on experiments with various sizes of fuel cell systems, with function varying in every system design. Accurate fuel concentration in such complex systems is a challenge. However, the environment condition will affect the fuel cell output power when fuel cell system was controlled at a specific concentration. Therefore, a method to control the system operating at lowest acceptable concentration and maximum output power with higher fuel efficiency was developed.